I. Field of the Invention
This invention relates generally to wireless communication networks such as, for example, cellular wireless local telephone systems and personal communication systems. More specifically, this invention relates to a novel and improved system and method for communicating information, in mobile cellular or satellite telephone systems, using spread spectrum type communication signals.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several methods for facilitating communications in systems accommodating a large number of users. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband are known in the art. However, CDMA spread spectrum modulation techniques have significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and is assigned to the assignee of this invention.
In U.S. Pat. No. 4,901,307 referenced above, a multiple access technique is disclosed where a large number of wireless system users each having a transceiver, communicate through satellite repeaters or terrestrial base station transceivers using CDMA spread spectrum communication signals. In using CDMA communication techniques, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplate the use of relatively long high speed pseudonoise (PN) sequences with each user channel being assigned a different PN sequence. The cross-correlation between different PN sequences and the autocorrelation of a PN sequence for all time shifts other than zero both have average values close to zero. In this way signals transmitted from a base station over a "forward" communication link are capable of being discriminated between upon reception by remote user or subscriber units.
However, because such PN signals are not completely orthogonal, over brief time intervals mutual interference noise is created between them. This interference noise arises despite the fact that the cross-correlations of the PN signals average to near zero, since for a short time interval, i.e., an information bit length, the cross-correlation follows a binomial distribution. As such, the forward link signals interfere with each other much the same as if they were wide bandwidth Gaussian noise at the same power spectral density. Accordingly, this type of mutual interference among signals transmitted over a forward link tends to limit achievable communication system capacity.
In U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", also assigned to the assignee of this invention, and which is incorporated herein by reference, there is disclosed a novel and improved method and system for constructing PN sequences that provides orthogonality between signals transmitted to subscribers over the forward link so that mutual interference is reduced. Such a reduction in mutual interference allows for higher system capacity and better link performance. Since using orthogonal PN codes make the cross-correlation zero over a predetermined time interval, no mutual interference arises, provided that the code time frames are aligned with each other.
In the system described in the just mentioned patent, a preferred waveform implemented involves using a direct sequence PN spread spectrum carrier. The chip rate of the PN carrier was chosen to be 1.2288 MHz in the preferred embodiment. One consideration involved in the choice of chip rate is that it be exactly divisible by baseband data rates to be used in the communication system. It is also desirable for the chip rate to be a power of two times the baseband data rate. In the preferred embodiment, the baseband data rate is 9600 bits per second, leading to a choice of 1.2288 MHz, which is 128 (2.sup.7) times 9600 for the PN chip rate.
In communications occurring between cellular base stations and various mobile units, the code sequences used for spreading the spectrum are constructed from two different types of sequences, each with different properties to provide different functions. There is an outer code that is shared by all signals in a cell or sector that is used to discriminate between multipath signals. The outer code is also used to discriminate between signals transmitted by different cells or sectors to the mobile units. There is also an inner code that is used to discriminate between user signals transmitted within a single sector or cell.
Implementation of a cellular CDMA system capable of providing adequate service to a particular geographic region generally involves consideration of a number of factors bearing upon system performance. For example, it is generally necessary to consider the extent of the available frequency spectrum, as well as the potential for coordination with other nearby communication systems. In addition, constraints imposed by thermal noise and interference generated by the various remote users or subscriber units needs to be taken into consideration. Estimates of interference are of particular concern within CDMA systems, since power is transmitted by the subscriber units over the same bandwidth irrespective of location within the cellular coverage area.
Interference on a forward, i.e., cell-to-subscriber, link can occur when base stations within neighboring cells use the same or an adjacent CDMA communication or radio channel as that intended for reception by a particular subscriber unit. In order to evaluate system performance under realistic conditions, a selected number of subscriber units may be deployed at various distances from multiple base stations as a means of estimating forward link interference levels. However, attempting to determine forward link interference through such field tests requires the availability of many multiple subscriber units and base stations. This would preclude complete system calibration when the number of base stations available during initial system testing was less than the number to eventually be installed.
Accordingly, it is an object of the present invention to provide a system for simulating interference due to signal transmission from base stations to subscriber units within a CDMA communication system.